nvmet-auth: complete a request only after freeing the dhchap pointers

It may happen that the work to destroy a queue
(for example nvmet_tcp_release_queue_work()) is started while
an auth-send or auth-receive command is still completing.

nvmet_sq_destroy() will block, waiting for all the references
to the sq to be dropped, the last reference is then
dropped when nvmet_req_complete() is called.

When this happens, both nvmet_sq_destroy() and
nvmet_execute_auth_send()/_receive() will free the dhchap pointers by
calling nvmet_auth_sq_free().
Since there isn't any lock, the two threads may race against each other,
causing double frees and memory corruptions, as reported by KASAN.

Reproduced by stress blktests nvme/041 nvme/042 nvme/043

 nvme nvme2: qid 0: authenticated with hash hmac(sha512) dhgroup ffdhe4096
 ==================================================================
 BUG: KASAN: double-free in kfree+0xec/0x4b0

 Call Trace:
  <TASK>
  kfree+0xec/0x4b0
  nvmet_auth_sq_free+0xe1/0x160 [nvmet]
  nvmet_execute_auth_send+0x482/0x16d0 [nvmet]
  process_one_work+0x8e5/0x1510

 Allocated by task 191846:
  __kasan_kmalloc+0x81/0xa0
  nvmet_auth_ctrl_sesskey+0xf6/0x380 [nvmet]
  nvmet_auth_reply+0x119/0x990 [nvmet]

 Freed by task 143270:
  kfree+0xec/0x4b0
  nvmet_auth_sq_free+0xe1/0x160 [nvmet]
  process_one_work+0x8e5/0x1510

Fix this bug by calling nvmet_req_complete() only after freeing the
pointers, so we will prevent the race by holding the sq reference.

V2: remove redundant code

Fixes: db1312dd ("nvmet: implement basic In-Band Authentication")
Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>

drivers/nvme/target/fabrics-cmd-auth.c

@@ -333,19 +333,21 @@ void nvmet_execute_auth_send(struct nvmet_req *req)
 			 __func__, ctrl->cntlid, req->sq->qid,
 			 status, req->error_loc);
 	req->cqe->result.u64 = 0;
-	nvmet_req_complete(req, status);
 	if (req->sq->dhchap_step != NVME_AUTH_DHCHAP_MESSAGE_SUCCESS2 &&
 	    req->sq->dhchap_step != NVME_AUTH_DHCHAP_MESSAGE_FAILURE2) {
 		unsigned long auth_expire_secs = ctrl->kato ? ctrl->kato : 120;
 
 		mod_delayed_work(system_wq, &req->sq->auth_expired_work,
 				 auth_expire_secs * HZ);
-		return;
+		goto complete;
 	}
 	/* Final states, clear up variables */
 	nvmet_auth_sq_free(req->sq);
 	if (req->sq->dhchap_step == NVME_AUTH_DHCHAP_MESSAGE_FAILURE2)
 		nvmet_ctrl_fatal_error(ctrl);
+
+complete:
+	nvmet_req_complete(req, status);
 }
 
 static int nvmet_auth_challenge(struct nvmet_req *req, void *d, int al)
@@ -514,11 +516,12 @@ void nvmet_execute_auth_receive(struct nvmet_req *req)
 	kfree(d);
 done:
 	req->cqe->result.u64 = 0;
-	nvmet_req_complete(req, status);
+
 	if (req->sq->dhchap_step == NVME_AUTH_DHCHAP_MESSAGE_SUCCESS2)
 		nvmet_auth_sq_free(req->sq);
 	else if (req->sq->dhchap_step == NVME_AUTH_DHCHAP_MESSAGE_FAILURE1) {
 		nvmet_auth_sq_free(req->sq);
 		nvmet_ctrl_fatal_error(ctrl);
 	}
+	nvmet_req_complete(req, status);
 }